Bio-integrated Electronics Research Articles

Nanoscale engineering of conducting polymers for emerging applications in soft electronics

Soft electronics featuring exceptional mechanical compliance and excellent electrical performance hold great promise for applications in soft robotics, artificial intelligence, bio-integrated electronics, and wearable electronics. Intrinsically stretchable and conductive materials are crucial for soft electronics, enabling large-area and scalable fabrication, high device density, and good mechanical compliance. Conducting polymers are inherently stretchable and conductive. They can be precisely synthesized from vastly available building blocks, and thus they provide a fruitful platform for fabricating soft electronics. However, amorphous bulk-phase conducting polymers typically exhibit poor mechanical and electrical characteristics. Consequently, it is highly desirable to develop novel engineering approaches to overcome the intrinsic limitations of conducting polymers. In recent years, numerous engineering strategies have been developed to enhance their performances in soft electronic devices via constructing various nanostructures. In this review, we first summarize several unique methodologies to fabricate conducting polymer-based nanostructures. We then discuss how nanoscale engineering approaches can improve several crucial parameters, including electrical conductivity, stretchability, sensitivity, and self-healing property of conducting polymers. Moreover, we also discuss device-level integration of conducting polymer-based nanostructures with other materials for applications in skin-inspired electronics and bio-integrated electronics. Finally, we provide perspectives on challenges and future directions in engineering nanostructured conducting polymers for soft electronics.

Nanoscale Materials and Deformable Device Designs for Bioinspired and Biointegrated Electronics

ConspectusElectronic devices whose structural and functional features are inspired by living creatures have unique performance and unconventional features that are not found in conventional electronic devices. In addition to such bioinspired electronics, with the rise of new fields such as personalized healthcare, mobile electronics, and big-data analysis, biointegrated electronic devices that can collect biomedical information from the human body through various biosensors and deliver appropriate therapeutic feedback stimulations in real time on the spot where immediate treatment is needed have become important. Because body parts and internal organs of living creatures, including humans, have curvilinear shapes and comprise mechanically soft tissues, such bioinspired and biointegrated electronic devices are required to match the soft and deformable features of biological tissues. Such soft and deformable features of electronic devices can be achieved by employing flexible and stretchable materials and unconventional device design techniques.These soft materials and deformable device designs dissipate stress originating from mechanical deformation of the device and thus retard crack generation and/or propagation in the device. Recently, technologies for nanoscale materials have shown a significant level of progress on their material performance and processing technologies. The nanoscale dimension of the electronic materials could achieve extremely small flexural rigidity in comparison to the bulk state of the same materials. Furthermore, techniques to form a well-percolated network of nanomaterials in the elastomeric matrix and to build a pathway for the facile electron and hole transport inside the polymer have induced dramatic performance advances of soft electronic materials, which led to nanocomposites that can accomplish both high mechanical deformability and high electrical performance at the same time. In addition, deformable device designs such as buckled structures and serpentine designs enhance the flexibility and stretchability of the device further. Because of their soft and deformable nature, bioinspired and biointegrated electronic devices could achieve device structures inspired by living creatures and make conformal contact to the target tissue for high-quality measurement of biological signals and real-time feedback treatments.Herein, we introduce recent advances in nanoscale materials and deformable device designs for bioinspired and biointegrated electronics. First, materials with various geometries (e.g., one-dimensional (1D) nanowires and nanotubes, two-dimensional (2D) nanomembranes and nanoflakes, and three-dimensional (3D) networks of nanomaterials in polymers) are reviewed in terms of their deformable nature. Then, the representative device design strategies required for achieving a soft and deformable form factor (e.g., buckling method, serpentine design, and kirigami technique) are reviewed. Examples of such state-of-the-art electronic devices are then presented, after which representative system-level applications, including electronic eyes, electronic skin, an electronic ear, wearable electronics, and implantable electronics, are described. Finally, we present a brief future outlook for the field of bioinspired and biointegrated electronics.

Physically Transient Diode With Ultrathin Tunneling Layer as Selector for Bipolar One Diode-One Resistor Memory

Herein, physically transient diode based on W/ZnO/MgO/W device with ultrathin MgO tunneling layer as selector for bipolar one diode-one resistor (1D1R) memory was demonstrated. By modulating thickness of MgO tunneling layer, the diode of ultrathin MgO (3 nm) tunneling layer exhibits stable rectifying behaviors, good endurance, high uniformity and a suitable reverse current. Meanwhile, the self-rectifying resistance switching functions of bipolar 1D1R memory that utilize reverse current of diode as compliance current are successfully obtained, which can suppress the sneak current of crossbar network. Importantly, the electrical feature of the transient 1D1R memory degraded after being immersed in deionized water (DI) for 1 min and completely triggered failure after 2 min. These results reveal that the transient 1D1R memory is promising for secure memory, neuromorphic computing and novel bio-integrated electronics systems.

Self-healing and stretchable PDMS-based bifunctional sensor enabled by synergistic dynamic interactions

The bio-integrated electronics industry is booming and becoming more integrated with biological tissues. However, it is challenging to obtain high-level self-healing of sensitivity and durability of bifunctional sensor when maintaining their long-term fatigue-resistance and high stretchability. Herein, a high performance self-healable bifunctional sensor was demonstrated through synergistic dynamic interactions between polysiloxane and silver nanoflakes (AgFs) and in-situ formed silver nanoparticles (AgNPs). The bifunctional sensor showed high sensitivity (gauge factor reached 39.5), exceptional electrical self-healability (at ambient temperature), and durability (more than 1000 cycles) by virtue of superflexible siloxane chains in the self-healing polymer, strong dynamic silver-thiolate coordination interactions, and the migration of silver nanoparticles. Finally, multiple human motions and health status were monitored, even after multiple cycles of healing, these “self-healed” sensing signals were still extremely consistent with the pristine ones, indicating the excellent reliabilities and healing stabilities of the bifunctional sensor.

Functionalized Elastomers for Intrinsically Soft and Biointegrated Electronics.

Elastomers are suitable materials for constructing a conformal interface with soft and curvilinear biological tissue due to their intrinsically deformable mechanical properties. Intrinsically soft electronic devices whose mechanical properties are comparable to human tissue can be fabricated using suitably functionalized elastomers. This article reviews recent progress in functionalized elastomers and their application to intrinsically soft and biointegrated electronics. Elastomers can be functionalized by adding appropriate fillers, either nanoscale materials or polymers. Conducting or semiconducting elastomers synthesized and/or processed with these materials can be applied to the fabrication of soft biointegrated electronic devices. For facile integration of soft electronics with the human body, additional functionalization strategies can be employed to improve adhesive or autonomous healing properties. Recently, device components for intrinsically soft and biointegrated electronics, including sensors, stimulators, power supply devices, displays, and transistors, have been developed. Herein, representative examples of these fully elastomeric device components are discussed. Finally, the remaining challenges and future outlooks for the field are presented.

Biocompatible, flexible and conductive polymers prepared by biomass-derived ionic liquid treatment

Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) is a promising, biocompatible conductive polymer for bio-integrated electronics with health-care applications.

Mechanics of Crater-Enabled Soft Dry Adhesives: A Review

Dry adhesion is governed by physical rather than chemical interactions. Those may include van der Waals and electrostatic forces, friction, and suction. Soft dry adhesives, which can be repeatedly attached to and detached from surfaces, can be useful for many exciting applications including reversible tapes, robotic footpads and grippers, and bio-integrated electronics. So far, the most studied Soft dry adhesives are gecko-inspired micro-pillar arrays, but they suffer from limited reusability and weak adhesion underwater. Recently cratered surfaces emerged as an alternative to micro-pillar arrays, as they exhibit many advantageous properties, such as tunable pressure-sensitive adhesion, high underwater adhesive strength, and good reusability. This review summarizes recent work of the authors on mechanical characterization of cratered surfaces, which combines experimental, modeling, and computational components. Using fundamental relationships describing air or liquid inside the crater, we examine the effects of material properties, crater shapes, air vs. liquid ambient environments, and surface patterns. We also identify some unresolved issues and limitations of the current approach, and provide an outlook for future research directions.

High-energy all-in-one stretchable micro-supercapacitor arrays based on 3D laser-induced graphene foams decorated with mesoporous ZnP nanosheets for self-powered stretchable systems

Micro-supercapacitors are promising energy storage devices that can complement or even replace lithium-ion batteries in wearable and stretchable microelectronics. However, they often possess a relatively low energy density and limited mechanical stretchability. Here, we report an all-in-one planar micro-supercapacitor arrays (MSCAs) based on hybrid electrodes with ultrathin ZnP nanosheets anchored on 3D laser-induced graphene foams (ZnP@LIG) arranged in island-bridge device architecture. The hybrid electrodes with a large specific surface area demonstrate excellent ionic and electrical conductivities, impressive gravimetric (areal) capacitance of 1425 F g −1 (7.125 F cm −2 ) at 1 A g −1 , and long-term stability. In addition to high energy (245 m Wh cm −2 ) and power (12.50 mW kg −1 at 145 m Wh cm −2 ) densities, the MSCAs with excellent cycling stability also showcase adjustable voltage and current outputs through serial and parallel connections of MSC cells in the island-bridge design, which also allows the system to be reversibly stretched up to 100%. Meanwhile, theoretical calculations validated by UV–vis absorption spectra partially suggest that the enhanced capacitance and rate capability may result from the improved electrical conductivity and number of adsorbed charged ions (Na + in Na 2 SO 4 aqueous electrolyte and K + in PVA/KCl gel electrolyte) on the pseudocapacitive non-layered ultrathin ZnP nanosheets. The integration of the all-in-one stretchable MSCAs with a crumpled Au-based triboelectric nanogenerator and stretchable crumpled graphene-based strain sensor demonstrates a self-powered stretchable system. The coupled design principle of electronic materials and device architecture provides a promising method to develop high-performance wearable/stretchable energy storage devices and self-powered stretchable systems for future bio-integrated electronics. • We report hybrid electrodes with ZnP nanosheets anchored on 3D laser-induced graphene foams. • The hybrid electrodes exhibit excellent ionic/electrical conductivities and long-term stability. • The hybrid electrodes with stretchable structures enable stretchable micro-supercapacitor arrays (MSCAs). • The MSCAs with adjustable voltage/current outputs can be reversibly stretched to 100%. • The integration of MSCAs with TENG and sensors demonstrates a self-powered stretchable system.

Mechanics of unusual soft network materials with rotatable structural nodes

Soft network materials consisting of filamentary horseshoe microstructures represent a class of architected materials of practical interest in flexible, bio-integrated electronics, because they can be tailored to reproduce accurately nonlinear stress−strain responses of biological tissues. The previous studies of these bio-inspired soft network materials focused mostly on the intriguing mechanical responses (e.g., J-shaped stress-strain curves and negative Poisson's ratios) before their fracture, and paid little attention on their utmost strength and stretchability. The development of soft network materials that can offer both a high stretchability (e.g., > 100%) and a relatively high strength (e.g., > 50 MPa) remains challenging. This paper presents the design, mechanics modeling and experimental measurement of a novel type of soft network materials that introduce rotatable structural lattice nodes (either with ring or disk shapes) to offer an enhanced deformability. Based on the analyses of the periodic building-block structure, we develop a nonlinear mechanics model of unusual soft network materials made of elastic constituent materials. Validated by finite element analyses (FEA) and experiments, this theoretical model can well predict the J-shaped stress-strain curves and deformed configurations of unusual soft network materials with a variety of geometric parameters. The results based on the developed model provide insights into the underlying relationship between nonlinear mechanical properties and key geometric parameters. For unusual soft network materials made of metals, an Ashby plot in terms of the strength and the stretchability is presented, highlighting the advantages of developed network materials over the existing network materials and typical soft materials. Moreover, computational models based on representative unit cells with periodic boundary conditions allow precise predictions of the utmost strength and stretchability, which can thereby serve as a reliable design tool. Due to the combined mechanical attributes of high stretchability, relatively high strength, and negative Poisson's ratio, the proposed unusual soft network materials can be used in various device systems of bio-integrated electronics.

3D-printed highly deformable electrodes for flexible lithium ion batteries

Flexible and stretchable batteries are the essential energy storage system to power wearable and epidermal electronics. A key component in flexible batteries is deformable electrodes that can sustain large and repeated stretches and maintain satisfactory electrochemical performance. Existing solutions to deformable electrodes often involve complex fabrication approaches, leading to high cost and thus limiting their widespread use. It is highly desirable to fabricate high performance deformable electrodes via a low-cost manufacturing approach. Here, we report 3D-printed LFP/LTO electrodes with satisfactory electrochemical performance and mechanical durability under large and repeated stretches. We show that, after 100 stretch-release cycles, 92% and 88% capacity retention could be obtained for the half cells with the suitably patterned and printed LFP cathode and LTO anode during 100 discharge/charge cycles. More importantly, a pouch cell battery with the suitably patterned and printed LFP/LTO electrodes exhibit a high discharge capacity of around 120 ​mAh g−1 at 0.3 ​C, as well as remarkable deformability. The facile 3D printing of the suitably patterned electrodes leads to low-cost manufacturing of high performance deformable electrodes, demonstrating the promising potential of such printed electrodes to enable stretchable and flexible energy storage devices to be used in soft robotics, wearable, and bio-integrated electronics.

Peptidoglycan-inspired autonomous ultrafast self-healing bio-friendly elastomers for bio-integrated electronics.

Elastomers are essential for stretchable electronics, which have become more and more important in bio-integrated devices. To ensure high compliance with the application environment, elastomers are expected to resist, and even self-repair, mechanical damage, while being friendly to the human body. Herein, inspired by peptidoglycan, we designed the first room-temperature autonomous self-healing biodegradable and biocompatible elastomers, poly(sebacoyl 1,6-hexamethylenedicarbamate diglyceride) (PSeHCD) elastomers. The unique structure including alternating ester-urethane moieties and bionic hybrid crosslinking endowed PSeHCD elastomers superior properties including ultrafast self-healing, tunable biomimetic mechanical properties, facile reprocessability, as well as good biocompatibility and biodegradability. The potential of the PSeHCD elastomers was demonstrated as a super-fast self-healing stretchable conductor (21 s) and motion sensor (2 min). This work provides a new design and synthetic principle of elastomers for applications in bio-integrated electronics.

Natural Lipid Inspired Hydrogel–Organogel Bilayer Actuator with a Tough Interface and Multiresponsive, Rapid, and Reversible Behaviors

Developing hydrogel-organogel bilayer films has greatly promoted the application of asymmetric structures in bionics, bio-integrated electronics, and other basic industries. However, the ability to...

Biomemristic Behavior for Water-Soluble Chitosan Blended with Graphene Quantum Dot Nanocomposite.

Bionanocomposite has promising biomemristic behaviors for data storage inspired by a natural biomaterial matrix. Carboxylated chitosan (CCS), a water-soluble derivative of chitosan avoiding the acidic salt removal, has better biodegradability and bioactivity, and is able to absorb graphene quantum dots (GQDs) employed as charge-trapping centers. In this investigation, biomemristic devices based on water-soluble CCS:GQDs nanocomposites were successfully achieved with the aid of the spin-casting method. The promotion of binary biomemristic behaviors for Ni/CCS:GQDs/indium-tin-oxide (ITO) was evaluated for distinct weight ratios of the chemical components. Fourier transform infrared spectroscopy, Raman spectroscopy (temperature dependence), thermogravimetric analyses and scanning electron microscopy were performed to assess the nature of the CCS:GQDs nanocomposites. The fitting curves on the experimental data further confirmed that the conduction mechanism might be attributed to charge trapping–detrapping in the CCS:GQDs nanocomposite film. Advances in water-soluble CCS-based electronic devices would open new avenues in the biocompatibility and integration of high-performance biointegrated electronics.

Mechanically and biologically skin-like elastomers for bio-integrated electronics

The bio-integrated electronics industry is booming and becoming more integrated with biological tissues. To successfully integrate with the soft tissues of the body (eg. skin), the material must possess many of the same properties including compliance, toughness, elasticity, and tear resistance. In this work, we prepare mechanically and biologically skin-like materials (PSeD-U elastomers) by designing a unique physical and covalent hybrid crosslinking structure. The introduction of an optimal amount of hydrogen bonds significantly strengthens the resultant elastomers with 11 times the toughness and 3 times the strength of covalent crosslinked PSeD elastomers, while maintaining a low modulus. Besides, the PSeD-U elastomers show nonlinear mechanical behavior similar to skins. Furthermore, PSeD-U elastomers demonstrate the cytocompatibility and biodegradability to achieve better integration with tissues. Finally, piezocapacitive pressure sensors are fabricated with high pressure sensitivity and rapid response to demonstrate the potential use of PSeD-U elastomers in bio-integrated electronics.

Material Design and Fabrication Strategies for Stretchable Metallic Nanocomposites

Stretchable conductive nanocomposites fabricated by integrating metallic nanomaterials with elastomers have become a vital component of human-friendly electronics, such as wearable and implantable devices, due to their unconventional electrical and mechanical characteristics. Understanding the detailed material design and fabrication strategies to improve the conductivity and stretchability of the nanocomposites is therefore important. This Review discusses the recent technological advances toward high performance stretchable metallic nanocomposites. First, the effect of the filler material design on the conductivity is briefly discussed, followed by various nanocomposite fabrication techniques to achieve high conductivity. Methods for maintaining the initial conductivity over a long period of time are also summarized. Then, strategies on controlled percolation of nanomaterials are highlighted, followed by a discussion regarding the effects of the morphology of the nanocomposite and postfabricated 3D structures on achieving high stretchability. Finally, representative examples of applications of such nanocomposites in biointegrated electronics are provided. A brief outlook concludes this Review.

Stretchability of PMMA-supported CVD graphene and of its electrical contacts

The remarkable mechanical robustness and excellent electrical/thermal properties make graphene a promising candidate for future flexible, stretchable and bio-integrated electronics. In practice, many soft electronics such as the graphene electronic tattoos (GETs) demand the chemical vapor deposited (CVD) graphene to be supported by a deformable substrate. Moreover, various conductive overlayers need to directly laminate on graphene to make electrical contacts. To investigate the mechanical reliability of CVD graphene in these situations, we fabricated CVD monolayer graphene supported by ultrathin poly(methyl methacrylate) (PMMA) substrate and also placed gold/polyethylene terephthalate (Au/PET) and graphene/PMMA (Gr/PMMA) overlayers on graphene. The stretchability of the Gr/PMMA and the overlayer-Gr/PMMA interface was characterized by electrical resistance change during uniaxial tensile tests. Combined with in situ microstructure and Raman investigation, we identified four deformation/fracture stages of Gr/PMMA—pre-cracking elastic deformation, limited micro-cracking in graphene, extensive cracking in graphene, and macro-cracking in PMMA. While micro-cracks emerged in graphene at very small strain (~0.9%), the electrical conductivity of the Gr/PMMA specimen remained up to tensile strains of ~14.5%. In contrast, 100 nm-thick Au film supported by the same PMMA substrate fully ruptured after tensile strains of ~1%. When laminating Au/PET and Gr/PMMA over Gr/PMMA, we found that the Au/PET- Gr/PMMA interface is very vulnerable but the Gr/PMMA- Gr/PMMA interface behaves very similar to intact Gr/PMMA electromechanically. The cyclic behaviour of Gr/PMMA, the effects of PMMA thickness and adhesion are also briefly discussed. The present experimental study provides fundamental insight into the failure of ultrathin polymer-supported graphene and its electrical contacts, which is critical for designing future graphene-based soft electronics.

High Resolution Micro-patterning of Stretchable Polymer Electrodes through Directed Wetting Localization

A microarray of conducting polymer electrodes with high resolution and high pattern-fidelity is developed on a stretchable substrate through the directed wetting localization (DWL) by the differential hydrophobicity. The large difference in the surface energy between the wetting and dewetting regions serves as the major determinant of the pattern resolution and the pattern-fidelity, yielding the full surface coverage in the stretchable electrode array (SEA) with 30 μm in width. The electrical characteristics of the SEA are well preserved under different types of elastic deformations. All-solution-processed polymer light-emitting diodes (except for the cathode) based on our patterned stretchable electrodes show no appreciable degradation of the performance under stretching. The DWL provides a simple and effective way of building up diverse stretchable electrical and optoelectronic devices in advanced wearable and bio-integrated electronics.

Custom-Made Electrochemical Energy Storage Devices

A customizable electrochemical energy storage device is a key component for the realization of next-generation wearable and biointegrated electronics. This Perspective begins with a brief introduction of the drive for customizable electrochemical energy storage devices. It traces the first-decade development trajectory of the customizable electrochemical energy storage devices. It then discusses the challenges and future directions, calling for such devices that allow users to select, design, and change the properties (including capacity, flexibility, shapes, and functionalities) according to real-life needs. Leveraging these customizable electrochemical energy storage devices will shed light on smarter programmable electrochemical energy storage devices to power future wearable and biointegrated electronics.

Ultrasoft electronics to monitor dynamically pulsing cardiomyocytes.

In biointegrated electronics, the facile control of mechanical properties such as softness and stretchability in electronic devices is necessary to minimize the perturbation of motions inherent in biological systems1-5. For in vitro studies, multielectrode-embedded dishes6-8 and other rigid devices9-12 have been widely used. Soft or flexible electronics on plastic or elastomeric substrates13-15 offer promising new advantages such as decreasing physical stress16-18 and/or applying mechanical stimuli19,20. Recently, owing to the introduction of macroporous plastic substrates with nanofibre scaffolds21,22, three-dimensional electrophysiological mapping of cardiomyocytes has been demonstrated. However, quantitatively monitoring cells that exhibit significant dynamical motions via electric probes over a long period without affecting their natural motion remains a challenge. Here, we present ultrasoft electronics with nanomeshes that monitor the field potential of human induced pluripotent stem cell-derived cardiomyocytes on a hydrogel, while enabling them to move dynamically without interference. Owing to the extraordinary softness of the nanomeshes, nanomesh-attached cardiomyocytes exhibit contraction and relaxation motions comparable to that of cardiomyocytes without attached nanomeshes. Our multilayered nanomesh devices maintain reliable operations in a liquid environment, enabling the recording of field potentials of the cardiomyocytes over a period of 96 h without significant degradation of the nanomesh devices or damage of the cardiomyocytes.

Recent Development of Flexible and Stretchable Antennas for Bio-Integrated Electronics.

Wireless technology plays an important role in data communication and power transmission, which has greatly boosted the development of flexible and stretchable electronics for biomedical applications and beyond. As a key component in wireless technology, flexible and stretchable antennas need to be flexible and stretchable, enabled by the efforts with new materials or novel integration approaches with structural designs. Besides replacing the conventional rigid substrates with textile or elastomeric ones, flexible and stretchable conductive materials also need to be used for the radiation parts, including conductive textiles, liquid metals, elastomeric composites embedding conductive fillers, and stretchable structures from conventional metals. As the microwave performance of the antenna (e.g., resonance frequency, radiation pattern, and radiation efficiency) strongly depend on the mechanical deformations, the new materials and novel structures need to be carefully designed. Despite the rapid progress in the burgeoning field of flexible and stretchable antennas, plenty of challenges, as well as opportunities, still exist to achieve miniaturized antennas with a stable or tunable performance at a low cost for bio-integrated electronics.